The invention relates to a bidirectional three-way star splitter for optical wave guides.
The existence of bidirectional star-splitters is generally a prerequisite for implementing fiber-optical bus systems for data transmission. They can be used, for example, to connect a subscriber device with bidirectional transmitting and receiving capability to a fiber-optic data line.
In German Published Unexamined Patent Application (DE-OS) No. 33 24 161, so called Y or T couplers are referred to in which two part luminous fluxes are combined into a single luminous flux via a gradient lens coupling element. In addition to this means of combining luminous fluxes, Y-couplers are also described in which two fibers are combined into a single fiber by fusing or bonding. The thermal softening, twisting and stretching of the fibers, however, requires considerable manual skill in production thereby excluding the possibility of automation. Additionally, not all connections of a Y-coupler can communicate with each other with the same level of priority. A bidirectional star splitter can be built only by using three Y-couplers as shown in FIG. 1 depicting the functional difference between Y-couplers and three-way star splitters.
In German Published Unexamined Patent Application (DE-OS) No. 33 24 611, a three-way star splitter is described in which the fiber is split up and a part of the light is coupled into another fiber by means of a reflective 90.degree. prism. The parallel beam path necessary for this is generated by three lenses as shown in FIG. 2.
Although this arrangement has the advantage that the degree of input and output coupling is continuously variable by moving the prisms, the high mechanical expenditure involved with the adjustability of the lenses prohibits mass production at a low price. Also for optically related reasons, a low-loss output coupling is possible only to approximately 50%, at least in the case of the three similar fiber connections as shown in FIG. 2. The advantage of the variable degree of output coupling can also be achieved by using an appropriately manufactured coupler having a fixed degree of output coupling.
An object of the present invention is to provide a bidirectional three-way star splitter for optical wave guides which is universally usable with low losses and can be simply and inexpensively produced in large numbers.
It is another object of the present invention to provide a star splitter which is combinable with other star splitters for implementing arbitrary bidirectional branching.
The above and other objects are attained by a three way star splitter arrangement having coupling elements positioned between the end front faces of a plurality of wave guides. These coupling elements each have a longitudinal inside surface coupled to a longitudinal outside surface at both ends by a planar coupler surface. The planar coupler surfaces extend parallel to the front end faces of the respectively adjoining optical wave guides. The planar coupler surface also forms at each end of the respective coupling elements, an enclosed acute angle with the respectively adjoining longitudinal inside surfaces.
In a particular preferred embodiment of the invention, the longitudinal inside surfaces are abuttingly joined to each other defining a line of intersection therebetween. These lines of intersection are aligned with the optical axis of the respectively adjoining optical wave guide.
The three-way star splitter of the present invention is particularly suitable for installation in inexpensive fiber-opticle bus systems. For this purpose, a specially preferred embodiment includes synthetic fibers for optical wave guides having diameters of 1 mm. Another preferred embodiment includes so-called thick-core fibers of glass or quartz for fibers having diameters of 0.5 mm. Coupling elements can be inexpensively produced in large numbers for these guides using injection molding methods. The surface quality achieved by these methods is adequate for data transmission purposes.
The present invention has the advantage of providing a light path in such a way that dividing and focusing of the beam of rays occurs completely inside a planar synthetic part (coupling element) having a homogeneous composition. The beam directions are influenced exclusively by the effect of total internal reflection at the interface between the synthetic part and the air. Surface treatment such as, for example, vapor deposition of reflecting layers thus becomes no longer necessary. If defraction effects are neglected, it is always possible because of the limited cone of radiation of the optical wave guide, to find points in the synthetic part which are located in the shadow, and can therefore be utilized for mounting the otherwise freely suspended synthetic part.
Since the inside and outside surfaces are substantially parallel to each other and the step of the refraction index between the synthetic part and air is always greater than that between the core and the sheath of the optical wave guide, total internal reflection always occurs in the vertical or transverse direction in the synthetic part. The propagation of the beams in the transverse direction is, therefore, limited to the height of the synthetic element which is, in a preferred embodiment, identical to the diameter of the core of the optical wave guide.
Since the reflection at the longitudinal surfaces of the coupling element is also intended to occur as total internal reflection, certain critical angles must be maintained. In this context, the elliptical reflector is designed to have an aperture angle of a cone of radiation of an incident light beam to be approximately equal to the aperture angle of a cone of radiation of the reflected light beam. Also the main object distance of the light rays is made to be approximately equal to the mean image distance. Additionally, the longitudinal inside surface of the coupling elements are located between the focal points of the respective elliptical reflection and the longitudinal outside surfaces of the respective coupling elements. The reflected focal points of the elliptical reflections are provided on the respective coupler surfaces. In a preferred embodiment, the numeric aperture for the optical wave guide is 0.53 and a refractive index, n, for the coupling element is 1.5.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, an embodiment in accordance with the present invention.